Hydrogen Crystallization in Low-Density Aerogels

Crystallization of liquids confined in disordered low-density nanoporous scaffolds is poorly understood. Here, we use relaxation calorimetry to study the liquid-solid phase transition of H-2 in a series of silica and carbon (nanotube- and graphene-based) aerogels with porosities greater than or similar to 94%. Results show that freezing temperatures of H-2 inside all the aerogels studied are depressed but do not follow predictions of the Gibbs-Thomson theory based on average pore diameters measured by conventional gas sorption techniques. Instead, we find that, for each material family investigated, the depression of average freezing temperatures scales linearly with the ratio of the internal surface area (measured by gas sorption) and the total pore volume derived from the density of aerogel monoliths. The slope of such linear dependences is, however, different for silica and carbon aerogels, which we attribute to microporosity of carbons and the presence of macropores in silica aerogels. Our results have important implications for the analysis of pore size distributions of low-density nanoporous materials and for controlling crystallization of fuel layers in targets for thermonuclear fusion energy applications.